In a known process for producing a ceramic sintered body having a metallized via hole, specifically a Cu-metallized glass ceramic substrate, for example, CuO, Cu, a mixture of CuO and Cu, or a mixture of CuO and Cu.sub.2 O is used as a raw material for Cu metallization. A green sheet is first sintered in the air to burn and remove an organic binder contained in the green sheet, followed by reductive sintering for reducing the CuO to Cu and then by sintering in a neutral atmosphere.
Further, a known process for producing a ceramic substrate having a straight via hole comprises punching via holes in green sheets, filling the via holes with a Cu paste, laminating a predetermined number of green sheets, and sintering the laminated sheets to produce a substrate having a predetermined thickness.
However, the former of the above-described two processes has had a drawback that since the oxidation and reduction of Cu results in fluctuations in the volume of the metallic filling material and this leads to cracking of the ceramic or the filling material, the raw material powder for metallization should be prepared by blending powders having proper particle sizes or various additives should be incorporated into the raw material powder.
The latter process, on the other hand, has had a problem that in laminating the sheets, it is difficult to position the sheets in such a manner that opposed via holes are precisely joined together and a problem that the metallic filling material formed from the Cu paste is apt to have impaired electrical conductivity at interfaces between sheets. Furthermore, there has been a problem that the amount of the Cu paste with which the via holes are to be filled should be regulated because too large amounts thereof result in cracking of the ceramic due to a volume increase of the filling material caused by Cu oxidation during binder-removing sintering, while too small amounts thereof result in diminished hermeticity or impaired electrical conductivity.
The present inventors have conducted extensive studies in regard to processes for producing a ceramic sintered body having a metallized via hole. As a result, it has been found that the above-described drawbacks of the prior art techniques can be eliminated by using a semi-sintered body having a via hole and filling only the via hole with a suspension for metallization without allowing the suspension to penetrate into pores of the semi-sintered body. The present invention has been completed based on this finding.